1. Field of the Invention
This invention relates to methods for improving the mask alignment in the fabrication of semiconductor integrated circuits that contain buried layers.
2. Description of the Prior Art
Many semiconductor integrated circuit technologies, including bipolar, JFET, CMOS and VMOS use buried layers. A buried layer is a semiconductor region in a substrate which is then covered by an epitaxial layer. Hence the buried layer is a region buried under the epitaxial layer. Each semiconductor device, as needed, may have its own buried layer.
For instance, the following steps are now known in the art to fabricate a buried layer integrated circuit, as shown in FIG. 1.
Starting with a silicon substrate 1 as shown in FIG. 1(a), its surface is oxidized to form a silicon dioxide layer 3 covering the principal surface of the substrate 1. Using conventional photoresist and masking methods, a window 5 is opened in the oxide layer 3. This window defines the location of the buried layer. Then, as shown in FIG. 1(b), the desired dopant is ion implanted through the window 5, to form buried layer 7. Then, as shown in FIG. 1(c), dopants are driven into buried layer 7 in an oxidizing ambient. Because of the different rates of oxidation between the exposed buried layer 7 and the surrounding silicon dioxide-covered area 3 during drive-in, a step 8 forms on the surface of the periphery of the buried layer 7. That is, the buried layer 7 is depressed below the surface of the substrate. This process is disclosed, for instance, in the textbook VLSI Technology, edited by S. M. Sze, McGraw-Hill, 1983, pages 51-53, 68-70.
It is important that the subsequent masking process be properly aligned with the buried layer 7. To achieve this, the depressed buried layer 7 must have a well defined edge with "step" 8 typically 0.05 to 0.1.mu. (500 to 1000.ANG.) high around its perimeter formed by differential oxidation. This perimeter "step" 8 marks the location of the buried layer 7 and is the result of the oxidation during the drive-in of the buried layer dopants.
The remaining oxide layer 3 is stripped off and the epitaxial layer 9 as shown in FIG. 1(d) is now grown on the surface of the substrate 1, over the buried layer 7. Since the epitaxial layer 9 is typically about 5.mu. thick (equal to 50,000.ANG.), the perimeter step 8 propagates through to the top surface of the epitaxial layer 9. Unfortunately, due to anisotropy, when the epitaxial layer is grown, the perimeter step is both shifted by distance x and distorted to an elongated shape shown in FIG. 1(f) (a top view along line f--f in FIG. 1(d)) relative to its original configuration shown in FIG. 1(e) (a top view along line e--e in FIG. 1(d)) in the substrate.
The shift and distortion complicate alignment of the subsequent mask. Specifically, the shift in location must be corrected for, and the distortion in the pattern shape makes recognition of the pattern difficult. Therefore, alignment requires a skilled human operator who can recognize the distorted perimeter step pattern, plus the complication of adjusting for shift. This obviates the use of automatic optical alignment equipment, and so makes processing slower and more expensive.
One improvement known in the art is to modify the epitaxial process. By growing the epitaxial layer at reduced pressure (lower than the usual atmospheric pressure), isotropic growth of the epitaxial crystal layer eliminates the distortion and shift. However, even with this improvement, it is still necessary to employ differential oxidation to drive-in the buried layer so that it includes a perimeter step and even then, current automatic alignment equipment does not readily recognize the rectangular step pattern.
Therefore, traditional prior art involves a buried layer step and anisotropic epitaxial growth. This produces a shifted and distorted step pattern, which is not recognizable by automatic alignment equipment, and which requires significant skill even of a human operator to recognize and correct for shift. The improved prior art would involve a buried layer step and isotropic epitaxial growth. This eliminates shift and distortion, but the rectangular step pattern still is not readily recognizable by automatic alignment equipment, and still requires a fairly skilled human operator.
It would be highly desirable to alter the process to eliminate the need for forming the perimeter step, and also make the process suitable for optical automatic alignment equipment such as projection aligners and steppers by creating a pattern easily recognized by such automatic equipment.